1. Field of the invention
This invention relates to an image display apparatus of the projection type.
2. Description of the prior art
In recent years, for the purpose of attaining a relatively large display of images with a compact, lightweight display apparatus, projection-type image display apparatuses that project the image on a screen have been used which employ a non-luminescent display panel of the transmission type to form the image by projecting light on the display panel from a light source behind the display panel.
The non-luminescent display panel does not emit light itself, but rather its transmittance is changed by a driving signal and the image is displayed by modulating the strength of the light from a light source provided separately. Examples of non-luminescent display panels are liquid crystal display panels, electrochromic display panels, and transmissive-type ceramic panels (e.g., display panels using lead lanthanum zirconate titanate (PLZT) in display elements). In particular, liquid-crystal display panels have been widely used in portable televisions, word processors, etc.
In these display panels, the smallest display units, referred to as picture elements, are arranged in a regular pattern and the image is displayed by the application of independent driving voltages to each picture element. The methods used to apply independent driving voltages to each picture element include the simple matrix drive system and the active matrix drive system.
For the purpose of obtaining a display of color images, a three-panel system is used which produces the color image by superposing monochrome images which are formed by three display panels corresponding to the three primary colors (i.e., red, green, and blue), or a one-panel system is used which performs a color display by arranging three kinds of color filters corresponding to the three primary colors in a mosaic or striped pattern (abbreviated to a color filter below) so as to face the respective picture elements of a display panel.
In the three-panel system, it is difficult to produce a compact, lightweight image display apparatus because of the needs for both the three display panels and an optical system for superposing the three monochrome images corresponding to the three primary colors.
On the other hand, it is relatively simple to produce a compact, lightweight image display apparatus and attain lower production costs with the one-panel system. However, in order to obtain a resolution equal to that of the three-panel system when colorizing the image with the one-panel system, three times as many picture elements are required for only one panel. Therefore, each picture element should be made smaller and the density of picture elements should be increased.
When using a liquid crystal display panel of a matrix-drive system with a color filter which transmits the light of the three primary colors (i.e., red, green, and blue) at each picture element, the switching elements and the various signal lines should be provided between the picture elements. Particularly, when using a liquid crystal display panel of an active-matrix drive system with switching elements (e.g., thin-film transistors or metal-insulator-metal (MIM) elements), separate display electrodes connected to these switching elements, driving signal lines which supply the driving signals to these display electrodes, and scanning signal lines which supply the control signals which scan the above switching elements. Therefore, the percentage per unit area (apertre ratio) of the area contributing to display (i,e., the area in which the display electrodes are formed) is decreased as compared to the area not contributing to display (i.e., the area in which the various signal lines and the switching elements are formed).
When displaying color images with a one-panel display system, it is necessary to make the display electrodes smaller and increase the density of picture elements as described above, so that the shading area which does not contribute to display becomes relatively large, thus further reducing the aperture ratio.
This decrease in the aperture ratio reduces the amount of light transmitted by the liquid crystal display panel, thus resulting in a darker display image even if the same light source is used.
To solve this problem, various methods have been disclosed in the Japanese Laid-open Patent Publications Nos. 60-165621 to 60-165624 and 60-262131, which improve the brightness of the display image by using a microlens array to converge the light incident upon the liquid crystal panel, on the picture element areas (i.e., display electrodes).
The following methods have been proposed for forming microlens arrays.
(1) A molding method in which a metal mold is used to form a synthetic resin material or glass into a microlens array. PA1 (2) A method in which convex lenses in a microlens array are formed by utilizing a phenomenon that when a specific type of photosensitive resin is exposed to light in a pattern corresponding to the microlens array, the non-reacted photosensitive resin moves from the non-exposed parts to the exposed parts and the exposed parts swells up to form the convex lenses. PA1 (3) A method in which convex lenses in a microlens array are formed by using a known photolithographic technique to pattern a thermoplastic resin in a plate shape corresponding to the microlens array and then heating the resin to a temperature above its softening point to give it flowability and cause curving at the edges. PA1 (4) A method in which convex lenses in a microlens array are formed by performing the proximity printing on a photosensitive resin and distributing the amount of photoreacted material according to the indistinctness of the transfer image on the photosensitive resin at the edges of the mask used in the proximity printing. PA1 (5) A method in which a configuration equivalent to a microlens array is obtained by irradiating light with an intensity distribution on a photosensitive resin to form a refractive index distribution corresponding to the intensity of the light. PA1 (6) A method in which a configuration equivalent to a microlens array is obtained by forming a refractive index distribution on glass or other transparent substrates using a selective ion diffusion technique. PA1 (7) A method in which convex lenses in a microlens array are formed by utilizing the contraction which accompanies crystallization when light is applied to a photosensitive glass.
Alternatively, methods in which a microlens array is combined with a color filter by selectively colorizing the microlens array (i.e., with the primary colors of red, green, and blue) using a pigment or dye have been disclosed in the Japanese Laid-open Patent Publications Nos. 61-208080 and 62-267791.
In the methods mentioned above, attempts should be made to control the spectral characteristics as the function of the color filter by selecting the shape thereof, but since the color filter also functions as a microlens array, it is difficult to change the shape which is established on the basis of the required lens characteristics. Also, since the transmittance varies according to the distribution of the thickness of the microlens itself (i.e., the transmittance decreases near the center of each microlens, whereas light in the wavelength band which should be absorbed is allowed to pass near the edge of each microlens), so that it is difficult to obtain the desired spectral characteristics. Moreover, the range of materials that can be used to form microlens arrays is relatively narrow from the viewpoint of such factors as refractive index, molecular structure, and melting point. Furthermore, since the materials that can be used to form microlenses are limited, the range of pigments and dyes that can be used is greatly limited. Therefore, it is desirable to provide a color filter separate from the microlens array.
When a microlens array is made using any one of the methods (1) through (7) above, the round micro-lenses are arranged with a predetermined spacing to prevent them from overlapping each other. In such a type of microlens arrays, there is a space remaining between the adjacent microlenses which does not contribute to convergence of light, so that all of the light incident upon the microlens array cannot be converged and used for display.
In view of this problem, it is thought that the converging capacity can be raised by changing the shape of each of the microlenses so that there is no space between them. For example, when the picture elements are arranged in an orthogonal lattice pattern, the microlenses can be packed together without space between them by giving each of the microlenses a rectangular shape corresponding to the picture element pitch.
In most cases, the picture elements of a liquid crystal display device are arranged in a striped matrix, diagonal matrix, or delta matrix. The microlenses are arranged according to the picture element matrix in the liquid crystal display panel. In the delta matrix, the picture elements in the odd and even numbered rows are mutually shifted half of the picture element pitch (in a color display panel, picture elements of the same color are shifted 1.5 , times the picture element pitch). The delta matrix has the advantages that the space resolution thereof has little anisotropy, the three primary colors are well mixed with each other, and the highest display quality is obtained in cases where an equal number of picture elements are used. Therefore, the delta matrix is employed in most portable liquid crystal televisions, and the same effect can also be obtained in the image display devices of the projection type.
When the picture elements are arranged in a delta matrix, the microlenses can be packed together with no space between them whether they are rectangular or hexagonal. When the microlenses are made using the method (1) above, the contour thereof can be divided into rectangles or hexagons without losing the rotational symmetry of the shape, so that there is almost no difference in the converging capacity between These microlenses.
However, when microlenses with a non-round contour are made using a method other than the method (1) above, by forming the exposure pattern into the desired shape, the rotational symmetry of the shape is lost, so that astigmatism occurs, thus increasing the size of or distorting the diameter of the convergence spot. If this causes the convergence spot to protrude from the opening of the picture element, then the converging capacity decreases. Other than the method (1) above, regardless of the method used to make the microlens array, the degree of the astigmatism is greater with rectangular lenses than with hexagonal lenses. This is because the degree of astigmatism changes according to the shift in the concave/convex shape of the microlenses or in their refractive index distribution from rotational symmetry.
Therefore, it is generally most effective to use a microlens array with a hexagonal contour for a liquid crystal display panel with picture elements in a delta matrix.
When forming a microlens array by the ion diffusion method (6) above so that there is no space between microlenses, the ions are diffused from a diffusion window much smaller than the diameter of the microlenses to be formed. Therefore, the shape of the microlenses naturally becomes hexagonal.
FIG. 8 shows the relationship between the shapes of conventional color filters and microlenses. In such a combination, the color filters 316R, 316G, and 316B are rectangular, so that white light or light of another color escapes from those parts protruding from the microlenses 314. This reduces the display contrast or the mixing of colors lessens the clarity, therefore, it is desirable to make the shape of the color filters the same as that of the microlenses.
In the case of image display devices having a liquid crystal display panel combined with a microlens array, both of the liquid-display panel and microlens array should be positioned near each other so that the microlenses in the microlens array individually corresponds to the picture elements in the liquid crystal display panel. Moreover, when performing a display of color images, alighnment is required between the color filter and the microlens.
However, in such an image display device, it is difficult to hold each of the parts at the respective predetermined position with high accuracy by mechanical means, and production costs are increased when attempts are made to increase accuracy. Moreover, if there is a layer of air between the liquid crystal panel, the microlens array, and the color filter, image quality will deteriorate because of loss arising from the interference, surface reflection, and scattering of light. To prevent loss arising from the interference, surface reflection, and scattering of light, these parts can be combined with each other by means of an adhesive.
In general, techniques for allowing two or more substrates to adhere together are used to combine optical parts with each other or to produce liquid crystal display devices. An ultraviolet setting resin is usually used to combine optical parts such as achromatic composite lenses, various types of prisms, and deflecting beam splitters. Although cold-setting resins are sometimes used, they are not suited to mass production because of their long setting time. In these optical parts, different kinds of glass are sometimes combined with each other, but there is little difference in their thermal expansion coefficients. Furthermore, there is no need to strictly control the thickness of the adhesive layer as long as it is kept thin.
Since liquid crystal display panels are required to have uniform electro-optical characteristics, the thickness of the panel should be constant. When combining the two substrates that constitute the liquid crystal display panel, spacers are first spread out between the substrates, and thereafter, liquid crystal is injected into the space between the substrates. It is also necessary to use spacers in the sealing resin, although the sealing portions are not required to transmit light. As the sealing resin, thermosetting epoxy adhesives are usually used in view of the effects on the characteristics and reliability of the liquid crystal display device. In the case of liquid crystal display panels, the substrates to be combined with each other are usually made of the same material, therefore, it is not necessary to consider differences in thermal expansion coefficients.
However, in cases where two substrates with different thermal expansion coefficients are combined with each other by conventional techniques, it is necessary to prevent relaxation of stress, warp and separation of the substrates due to the changes in the thermal environment. Particularly, in cases where a microlens array is combined with a liquid crystal display panel by means of an adhesive, the adhesive will cause changes in the optical characteristics or the non-uniform thickness of the adhesive will cause shifts in the focal distance, so that the effect of prevention of interference, surface reflection, and scattering of light is reduced.
For example, the problems will be discussed which occur in cases where a liquid crystal display panel with borosilicate glass substrates is combined with a flat microlens array with a soda-lime glass substrate.
As mentioned above, the microlens array converges the light incident upon the black matrix portion of the liquid crystal display panel, on the picture elements, so that the brightness of the display image is increased and the equivalent aperture ratio is raised. For the purpose of attaining the greatest possible effect, the microlens array should be combined uniformly with the liquid crystal display panel by adjusting the thickness of the adhesive therebetween so that the focal point of each of the microlenses is positioned in the area of the corresponding picture element.
In the case of a flat microlens array given a refractive index distribution by means of an ion-exchange method, a soda-lime glass containing a great amount of sodium ions is used as the substrate. Since soda-lime glass is not desirable as the substrate of liquid crystal display panels due to the elution of ions to the liquid crystal, which degrades the characteristics of the liquid crystal, borosilicate glass which has a low ion content is used. Since the thermal expansion coefficient of soda-lime glass is 8-10.times.10.sup.-6 deg.sup.-1 and that of borosilicate glass is 4-5.times.10.sup.-6 deg.sup.-1 when a pair of three-inch substrates are combined with each other, a warp of about 0.9 mm will occur at a temperature of 150.degree. C., therefore, the reliability with respect to heat is decreased.